The Mars Atmosphere and Volatile EvolutioN mission studies the current Martian atmosphere in order to learn more about today’s atmosphere and to take a look into the past. As surface missions have shown, Mars once had a thicker atmosphere and was warm enough for liquid water to flow on its surface.
Somehow, that thick atmosphere was lost to space – overall Mars has most likely lost 99% of its original atmosphere over a period of several million years as a result of its core cooling down and its magnetic field decaying. Without an active magnetic field, Mars was vulnerable to the solar wind that slowly swept away most of the water and volatile compounds of the atmosphere – turning Mars into a deserted planet.
MAVEN’s big goal is to determine the history of the loss of atmospheric gas to space and with that, provide insight into the evolution of Martian climate. Measuring the current rate of atmospheric loss and examining the exact processes that are behind it will allow scientists to infer how the atmosphere evolved over time.
MAVEN has four primary science objectives:
- Determine the role that loss of volatiles to space from the Mars atmosphere has played through time.
- Determine the current state of the upper atmosphere, ionosphere, and interactions with the solar wind.
- Determine the current rates of escape of neutral gases and ions to space and the processes controlling them.
- Determine the ratios of stable isotopes in the Martian atmosphere.
Previous missions to Mars have established that Mars lost the majority of its atmosphere over the course of the planet’s evolution. Rover missions have found evidence of ancient riverbeds and minerals and rocks that formed in the presence of flowing water – indicating that Mars once had a thicker, warmer atmosphere with abundant water and volatile contents.
Measurements made by Mars Express and Phobos have shown energetic ions moving away from the planet. Mars Global Surveyor data led scientists to infer a decrease in upper atmospheric density in response to a Solar Energetic Particle event.
Scientists hypothesize that Mars became vulnerable to escape processes about 3.7 billion years ago when its planetary core cooled down and caused the planet’s magnetic field to decay. Currently, Mars only shows crustal magnetic fields that produce mini-magnetospheres that influence the interaction of the current Martian atmosphere with the solar wind.
Those mini-magnetospheres are studied by MAVEN using its Magnetometer instrument. Without a global magnetosphere, Mars began a long process of atmospheric loss.
Atmospheric loss can be caused by a number of factors including photochemistry as dissociative recombination reactions of ions such as O2+, N2+ or CO+ with electrons create hot (energetic) particles with sufficient energy to escape the atmosphere. Another scenario for atmospheric loss is pick-up ionization – ions are produced through photoionization or charge exchange processes and are dragged along by solar wind to partially escape.
Ionospheric outflows can be generated when planetary ions are accelerated by Solar Wind induced electromagnetic fields in the ionosphere causing the ions to be lost in the wake. The Ion Sputtering theory suggests that a portion of Solar Wind and pick-up ions impact the neutral atmosphere with enough energy to eject neutrals at/above the exobase for example Oxygen, Carbon Monoxide, Carbon Dioxide, Nitrogen, Carbon, etc.
To study atmospheric escape, MAVEN measures ions at low energies (non-escaping) to high energies (escaping/sputtering). Using its Particles and Fields Package, MAVEN tracks ions down to the exobase in order to examine driving loss processes. MAVEN makes measurements of all charged particles populations with improved instruments and methods with high angular resolution, energy range and pitch angle measurements.
In addition to directly tracking loss processes, MAVEN characterizes the current state and properties of the Martian atmosphere. Using its instruments, the spacecraft measures upper atmospheric density, composition and structure as well as local and temporal variability. In situ measurements by the Neutral Gas and Ion Mass Spectrometer provide information on upper atmospheric composition and the Imaging Ultraviolet Spectrograph can use its stellar occultation measurements to determine Carbon Dioxide abundance and atmospheric density deep into the Martian atmosphere. MAVEN also determines the thermal state of the Martian ionosphere.
Another important factor of MAVEN’s research is the simultaneous measurement of energy inputs through extreme ultraviolet radiation, electromagnetic waves, Solar Wind, and Solar Energetic Particles to correlate escape processes and variability with solar changes.
NGIMS measures the profiles of Helium, Oxygen, Nitrogen, Carbon Monoxide, Carbon Dioxide, Nitrogen Monoxide and Argon as well as their major isotopes to determine accurate isotope ratios that can provide more information on atmospheric evolution. NGIMS and the Sample Analysis at Mars Instrument aboard the Curiosity Rover work in cooperation to provide composition data from two vantage points – the Martian surface inside the ‘thicker’ atmosphere and from within the upper atmosphere.
SAM already provided confirmation on atmospheric loss processes via isotope ratio measurements. Argon, one of the noble gases, accounts for 1.6 percent of the Martian atmosphere. The ratio of two isotopes of Argon, the heavier Ar38 and the lighter Ar36 provided additional data on the atmospheric loss suffered by Mars. Earlier, the ratio of Deuterium and Hydrogen was determined and also provided evidence for a major atmospheric loss.
Because Argon is a non-reactive gas, its isotope ratio provides more robust data as Argon is not involved in any chemical reactions that could change the isotope ratios. Also, because of its higher atomic weight, Argon ratios represent more conclusive evidence for atmospheric escape than the D/H ratio.
SAM found that a significant amount of Ar36 was missing which supports the theory of atmospheric loss that favors lighter elements. The Ar36/Ar38 ratio as determined by SAM is 4.19 (+/-0.035). This Argon ratio is much lower than the solar system’s original ratio of 5.5 that was determined through measurements of the sun and Jupiter as well as Earth.
SAM also analyzed Carbon and Oxygen isotope ratios and concluded that these elements are also rich in heavy isotopes. Due to physical loss processes, lighter elements/isotopes are favored while heavier elements are retained because of gravity. This produces a difference in isotope abundances is over large time scales of hundreds of millions of billions of years.
Measurements by Instrument
Langmuir Probe and Waves:
- Measures electron energy and number density throughout the upper atmosphere
- Electric field wave power at low frequencies that is important for ion heating which could lead to ion escape or sputtering
- Acquisition of wave spectra of naturally emitted and actively stimulated Langmuir waves to calibrate density measurements
- Measures the solar Extreme Ultraviolet irradiance at wavelengths important for ionization, dissociation and heating of the upper atmosphere
- EUV measured with high temporal resolution to study EUV variability
- Generate a full solar EUV spectrum using limited resources
Imaging Ultraviolet Spectrometer:
- Neutral Atmosphere & Corona: Profiles and column abundance of Hydrogen, Carbon, Nitrogen, Oxygen, Carbon Monoxide, Carbon Dioxide from the homopause up to two scale heights above the exobase
- Ionosphere: Profiles and column abundances of C and CO ions from the ionospheric main peak to the nominal ionopause
- Isotopes: Deuterium/Hydrogen ratio above the homopause with sufficient accuracy to examine spatial and temporal variations and compare with SAM D/H ratios
- Lower Atmosphere: Carbon Dioxide Profiles using stellar occultation measurements to characterize the underlying atmosphere
- Disk maps near apoapsis to characterize spatial distribution and variability
- D/H and Oxygen coronal mapping to quantify escape processes
Neutral Gas and Ion Mass Spectrometer:
- Examine the structure and composition of the upper Neutral atmosphere, measure thermal and supra thermal ions
- Basic Structure of the upper atmosphere and ionosphere from the homopause to above the exobase (Helium, Nitrogen, Oxygen, Carbon Monoxide, Nitrogen, Nitrogen Monoxide, Argon, Carbon Dioxide)
- Detect isotope ratios and variations (Carbon, Oxygen, Nitrogen, Argon)
- Over mission duration: study changes in atmosphere with perturbations from above and below
- Provide data to establish models of present and past atmospheric loss and a better understanding of the history of the Martian climate
- Measures MAVEN orientation data and all accelerations acting on the spacecraft
- Atmospheric density and vertical profiles studying local and temporal variability with respect to external processes
- Temperature profiles
- Wind profile measurements
- Measure vector magnetic fields in the unperturbed solar wind, magnetosheath and crustal magnetospheres with the ability to resolve crustal magnetic cusps
- Characterization of solar wind interaction
Support Particle & Fields Package
- Crustal Magnetospheres: day/night asymmetry in solar wind interaction, variability with solar input particularly variations in solar wind ram pressure, influence on loss of volatiles, geologic history
Solar Wind Electron Analyzer – SWEA:
- Measurements of energy and angular distribution of electrons in the Martian environment
- Electron impact ionization rates – Magnetic pileup region & ionosphere
- Magnetic topology via loss cone measurements
- Primary ionospheric photoelectron spectrum
- Auroral electron populations
- Mars plasma environment
- Draped field lines
Solar Wind Ion Analyzer – SWIA:
- Density and velocity distributions of Solar Wind and magnetosheath ions
- Determine the charge exchange rate and the bulk plasma flow from solar wind speeds down to stagnating magnetosheath speeds
- Measure total solar wind input with high temporal resolution
- Study basic space plasma processes around Mars
Solar Energetic Particle – SEP:
- Characterization of solar particles at energies that affects upper atmospheric and ionospheric processes
- Ions from 25keV to 12 MeV, Electrons from 25keV to 1MeV
Suprathermal and Thermal Ion Composition – STATIC:
- Escaping ions and processes
- Composition of thermal to energetic ions – energy distribution, pitch angle variations
- Ionospheric Ions (0.1-10eV), Tail suprathermal ions (5-100eV), Pick-Up ions (100-20,000eV)